Fix invalid escape sequence warnings #11234

Job	Run time
rip-and-test-clang	26m 23s
htmldocs	24m 47s
cppcheck	2m 5s
rip-and-test	26m 28s
package-arch (debian:bullseye)	28m 50s
package-indep (debian:bullseye)	42m 57s
package-arch (debian:bookworm)	29m 46s
package-indep (debian:bookworm)	33m 19s
package-arch (debian:sid)	32m 20s
package-indep (debian:sid)	39m 8s
	4h 46m 3s

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